Regular Expressions Basics: Master Text Matching from Scratch

What are Regular Expressions?

Regular expressions (Regex) are tools for describing string matching patterns. They can be used to:

Validate input formats (email, phone numbers, etc.)

Search and replace text

Extract specific information

Data cleaning and processing

Basic Syntax

Literal Characters

The simplest regular expressions are ordinary characters that match themselves:


hello  matches "hello"
123    matches "123"



Special Characters (Metacharacters)

Character Meaning

. Match any single character
^ Match start of string
$ Match end of string
Match preceding character 0 or more times
+ Match preceding character 1 or more times
? Match preceding character 0 or 1 time
\ Escape special characters

Character Classes


[abc]    Match a, b, or c
[a-z]    Match any lowercase letter
[0-9]    Match any digit
[^abc]   Match any character except a, b, or c



Predefined Character Classes


\d     Match digits, equivalent to [0-9]
\D     Match non-digits
\w     Match word characters (letters, digits, underscore)
\W     Match non-word characters
\s     Match whitespace characters
\S     Match non-whitespace characters



Quantifiers


{n}      Match exactly n times
{n,}     Match at least n times
{n,m}    Match n to m times



Grouping and Capturing

Use parentheses () to create capture groups:


(abc)+      Match one or more "abc"
(\d{4})-(\d{2})-(\d{2})    Match date format



Common Regular Expression Examples

Email Validation


^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$



Phone Number Validation (China)


^1[3-9]\d{9}$



URL Matching


https?:\/\/[\w\-]+(\.[\w\-]+)+[\w\-.,@?^=%&:/~+#]



Chinese Characters


[\u4e00-\u9fa5]+



Using Regular Expressions in JavaScript

`javascript


// Create regular expression
const regex = /hello/i;  // i means case-insensitive

// Test match
regex.test('Hello World');  // true

// Search match
'Hello World'.match(/\w+/g);  // ['Hello', 'World']

// Replace
'Hello World'.replace(/World/, 'Regex');  // 'Hello Regex'

Using EfficTools to Test Regular Expressions

The best way to learn regular expressions is through practice. Using our Regular Expression Testing Tool, you can:

Test regular expressions in real-time

View highlighted match results

View capture group contents

Support for multiple flags

Learning Tips

Start simple and gradually increase complexity

Practice more and test more

Use online tools to verify

Read others' regular expressions to learn techniques

Summary

Regular expressions are a skill worth investing time in learning. Although they may seem complex at first, once you master the basic syntax, they will become your powerful assistant for text processing.